Contact microphone



July 4, 1939.

. s. BALLANTINE CONTACT MICROPHONEA Filed Aug. 13, 1957 .SOKAQ k als o0Patented July 4, 1939 UNITED STATES PATENT OFFICE Application August 13,

14 Claims.

'Ihis invention relates to improvements in contact microphones of themoving-armature type, as for example the throat microphone described inmy copending application Serial No. 74,201, filed April 13, 1936 nowPatent No.. 2,121,781.

In this type of microphone, the mechanical vibrations of the source arecommunicated to an armature and cause it to vibrate in relation to afixed pole piece, thus varying the magnetic flux and generating anelectrical voltage between the terminals of a suitable coil. Thesensitivity of the device (defined as the voltage generated per unitvelocity of the armature) depends upon a number of factors, forexample: 1) number of turns on the coil, (2) strength of steadymagnetization produced either by a permanent magnet or by D. C. currentthrough the coil, (3) reluctances of the magnetic materials; and (4)length of the air-gap between armature and pole piece.

The pressure with which the microphone is applied to the source ofvibrations will cause a steady deilection of the armature, and theWorking air gap and sensitivity will depend upon this pressure. For manyuses to Which a contact microphone may be put, as for example its use asa throat microphone, this increase in output with applied pressure is aserious disadvantage. In the case of a throat microphone, it is verydinicult to maintain initially applied or steady pressure at a constantvalue because of variations due to movements of the head, etc.

The term steady pressure, as employed in the specification and claims,indicates the pressure of engagement of the microphone and the source ofmechanical vibrations when the source is at rest and, as just statedwith reference to throat microphones, this steady pressure may varyduring the use of the microphone.

In my experimental Work With the throat microphone, I have discoveredanother effect which makes this increase of microphone response withpressure doubly undesirable. This is the effect of compression of thetissues of the neck in increasing their ability to transmit the soundvibrations from the thyroid cartilage, or voice box. The output of themicrophone will increase as the pressure of application increases due tothis effect alone, even though the microphone sensitivity Were notaffected by pressure per se. Hence, in the case oi the throat microphonethere are tvvo effects that operate to cause undesirable variation inoutput with pressure.

Objects of this invention are to provide contact microphones of themoving armature type, and methods of operating such microphones, that1937, Serial No. 158,992

are characterized by a reduction in the variation in output with appliedpressure. An object is to provide throat microphones of the movingarmature type in which the output decreases with applied pressure, thuscompensating or tending to compensate for the increase in transmissionefficiency of the tissues oi the neck as the microphone pressure isincreased. More specically, an object is to provide throat microphonesof the moving armature type in which the reluctance of the magneticcircuit and the coercive force of the permanent magnetization are sorelated as to produce saturation or incipient saturation in the magneticcircuit, thus precluding an increase in output with a decrease in theair gap.

These and other objects and advantages of the invention Will be apparentfrom the following specification when taken with the accompanyingdrawing in which:

Fig. l is a curve showing the variation in sensitivity With length ofair gap in a typical contact microphone of the moving armature type;

Fig. 2 is a side elevation of the principal elements of a throatmicrophone embodying the invention;

Fig. 3 is a curve sheet showing the relation of magnetic force tomagnetic flux in the iron parts of the magnetic circuit; and

Figs. 4 and 5 are curve sheets illustrating the relation between outputand air gap for certain embodiments of the invention.

The relation between microphone output and length of air gap for atypical contact microphone of the moving armature type is illustrated bycurve A of Fig. 1 The magnitude of the output is quite small,particularly in the case of the throat microphone, and the designpractice developed in other branchesof the telephone receiver andmicrophone arts Would indicate that the gap should be made as short aspossible. 'I'he output for an operating point corresponding to about0.0015 inch is almost double the output that is obtained when the gap isincreased to about 0.003 inch.

A simpliiied contact microphone construction conforming to thisinvention, as shown in Fig. 2, has a movable armature I forming a partof the magnetic circuit that includes the pole piece 2, connecting bar 3and permanent magnet 4. A coil 5 of ine Wire is mounted on the polepiece 2 and has terminals 6 for connection to a transmission line. Oneend of the armature l is secured to the magnet 4 and the other endcarries a button 'l for engagement With a source of Vibrations 8 thatis, in the case of a throat microphone, the throat of the operator.Vibrations imparted to the armature I by the source 8 produce variationsin the length of the air gap 9, thus varying the magnetic flux throughthe pole piece 2 and consequently inducing electrical voltages in thecoil 5.

In accordance with this invention, the operating characteristics of themicrophone .are such as to reduce or to eliminate the variation ofoutput With changes in contact pressure. Means for predetermining or foradjusting the operating characteristics are illustrated in Fig. 2 as thenotch IIJ in armature I for establishing a predetermined ux density atthe air gap; the clamp bolt II for securing the pole 2 in desiredadjustment on bar 3 to determine the length of the air gap; and theadjustable magnetic shunt I2 that is slidably mounted on bar 3. Themanner in which one or more of these elements may be used to control theoperating characteristics Will be clear from the following explanation.

The output voltage e of a system such as shown in Fig. 2, for a givenvelocity 1J of the armature, can be expressed as:

Where n is the number of turns on the coil, qb is the total magneticflux and .1: is the displacement of the armature. For a given number ofturns on the coil the sensitivity is proportional to the factor dqs/d,that is to the change in flux for unit displacement of the armature. Nowif the air gap be reduced in length, as for example by a steadydeflection of the armature produced by the pressure of contact againstthe throat, the reluctance of the air gap is reduced and more magneticux flows through the magnetic circuit. At the same time the change inreluctance (and also the change in iiux) for a given displacement of thearmature increases, thus increasing the sensitivity, as shown in Fig. 1.This, as pointed out above, is the conventional method of operation ofthe movable armature type of microphone and is undesirable in the caseof contact microphones.

According to this invention, the magnetic circuit is so designed that,With the normal air gap, it is in a state of incipient saturation;whereby the decrease in air gap reluctance Which results from a furtherdecrease in the air gap isY automatically compensated or overcome by theincrease in reluctance of the iron parts of the magnetic circuit due tothe onset of saturation. The total reluctance thus tends to remainconstant and the sensitivity likewise tends to remain constant. The Waythis automatic compensation by saturation occurs may be illustratedgraphically as follows.

Let us assume that the magnet 4 produces a certain steady magnetizationforce I-Io. Let H=f() represent the relation between ux qa and magneticforce H in the ferrous parts of the magnetic circuit. Then the variationof flux with air gap can be found graphically as shown in Fig. 3. Thecurve OA represents H :f(q or the characteristic of the iron parts ofthe magnetic circuit. Now H0:Ra+f( Where Ra is the reluctance of the airgap (zlength/area), hence qa is given by the intersection of the curveOA and the curve BC Which represents Hin-Raga As the length of the airgap varies, due to vibration of the armature, this line BC Will Varybetween the extremes represented by the dotted lines BC, BC. The changein ilux will follow the small incremental ellipse a, b and the voltageoutput Will be proportional to Aqb. As the air gap is decreased bypressure against the throat or other source the angle e, Fig. 3,increases and the operating point on curve OA moves from point D intothe region of saturation E as shown by the dashed lines B1', B1, B1. Itwill be seen that the slope of the (p, I-I curve OA of the magneticcircuit is now less than before due to saturation, and Aqa and theoutput voltage for a given armature displacement remains constant ordecreases. We may place the operating point D above the point ofinflection F at the upper knee of the curve, or just below it, so as toobtain substantially constant output as the air gap is furtherdecreased, or may, by taking the operating point further into the regionof saturation, actually obtain a decrease in output with decreasing airgap.

According to the conventional method of operation, the `air gap would beshortened as much as possible in order to secure maximum sensitivity, Astypical operating point would be as shown in Fig. 1. I depart from thisprocedure and widen the air gap to a value such that deflection of thearmature Will not cause the complete closure of the gap at the maximumpressure of application likely to be encountered. This entails adeliberate sacrice of sensitivity but is desirable in view of theadvantages that may be obtained when the magnetic circuit is designed oradjusted to exhibit the desired saturation characteristics.

The desired magnetic saturation can be brought about in practice inseveral Ways: (l) by increasing the strength of the permanent magnet bythe use of alloys having a high coercive force, for example, Alnico; (2)by decreasing the thickness of the armature, or decreasing thecross-section of the magnetic path at some other point, and (3) by theuse of high-permeability materials e. g., nickel-iron alloys) in somepart of the magnetic circuit, for example in the armature. The'secondmethod is illustrated in Fig. 2, the desired decrease in section beingproduced by a constriction at groove I where saturation takes place.

If the design permits, the adjustment of the coercive force of thepermanent magnet offers a very convenient method of obtaining a properdegree of saturation. An alternative method is to select a suitablealloy for the armature. In this connection, I have carried the inventioninto use by substituting a nickel-iron alloy such as Allegheny electricmetal for soft iron or silicon steel as an armature material. For anygiven magnetization, the adjustability of pole piece 2 and magneticshunt I2 permit control of the degree of saturation.

The types of output versus air gap characteristic which I obtain by thismethod are illustrated by curves I3, Irl of Figs. 4 and 5. In Fig. 4,the operating point is at A (normal air gap=0.0035 in.) and furtherreduction in the air gap by pressure against the source results inpractically no change in sensitivity. When the output characteristic isof the type shown by curve I4 of Fig. 5, a reduction in air gap bypressure results in an actual decrease in output due to saturation ofthe armature. The latter characteristic is especially adapted to offsetthe increase of transmitting eiiiciency of the neck with increasingpressure When the device is to be used as a throat microphone. It willbe seen that both characteristics represent considerable improvementover the conventional prior art characteristics shown in Fig. 1.

Curve I 3 of Fig. 4 is an actual experimental curve taken on anelectromagnetic microphone of the type described in my copendingapplication Serial No. 74,201. In this case, the armature was of siliconsteel .014 thick and the permanent magnets were 55g long made of Alnico.'I'he magnets were magnetized after assembly by the application of 4.6amperes through a 100 turn coil. Curve I4 of Fig. 5 was obtained with anarmature .014" thick of Allegheny electric metal (nickel-iron alloy)using Alnico magnets of the same dimensions.

A characteristic of the type of Fig. 4 has the further importantpractical advantage of reducing variation in output due to setting ofthe air gap during manufacture as it renders the setting of the initialair gap much less critical.

As an example of the benefit thus derived, I

may say that out of 1000 microphones incorporating this principlerecently manufactured only 4 (0.4 per cent) were rejected because ofvarie-.tion in output.

According to the conventional method of operation, the air gap would beshortened as much as possible in order to secure maximum sensitivity. Atypical operating point would be as shown in Fig. l. I depart from thisprocedure and Widen the air gap to a value such that deflection of thearmature will not cause the complete closure of the gap at the maximumpressure of application likely to be encountered. This entails adeliberate sacrifice of sensitivity but is desirable in view of theadvantages that may be obtained when the magnetic circuit is designed oradjusted to exhibit the desired saturation characteristics.

It is to be understood that the iield or normally constant magnetizationin the magnetic circuit may be supplied either by a permanent magnet, bya steady current through the coil, or other means and that Where theadjustment of permanent magnetism is spoken of in the following claimsit is contemplated that this adjustment can be carried out by adjustingthe dimensions of a permanent magnet or its magnetization, orequivalently by adjusting the current through the coil.

I claim:

l. The method of maintaining the sensitivity of a Contact microphonesubstantially independent of changes in the length of the air gap in themagnetic circuit that result from variations in the steady pressure withwhich the microphone is applied to a source of mechanical vi'- brations,the magnetic circuit including a permanent magnet in series with the airand an armature that is vibrated by the source to alter the flux in themagnetic circuit, said method comprising adjusting the normal air gapand the coercive force of the permanent magnet to produce incipientsaturation of the magnetic circuit thereby to prevent an increase insensitivity when the microphone is pressed against the source ofvibrations so strongly as to reduce the air gap below its normal value.

2. The method of maintaining the sensitivity of a contact microphonesubstantially independent of changes in the length of the air gap in themagnetic circuit that result from variations in the steady pressure withwhich the microphone is applied to a source of mechanical vibrations,the magnetic circuit including a permanent magnet in series with the airgap and an armature that is vibrated by the source to alter the flux inthe magnetic circuit, said method comprising adjusting the normal airgap and the coercive force of the permanent magnet to produce saturationof the magnetic circuit thereby to prevent an increase in sensitivityWhen the microphone is pressed against the source of vibrations sostrongly as to reduce the air gap below its normal value.

3. The method of maintaining the sensitivity of a contact microphonesubstantially independent of changes in the length of the air gap in themagnetic circuit that result from variations in the steady pressure Withwhich the microphone is applied to a source of mechanical vibrations,the magnetic circuit including a permanent magnet in series with the airgap and an armature that is vibrated by the source to alter the flux inthe magnetic circuit, said methd comprising magnetizing the permanentmagnet to provide a coercive force at the normal air gap which placesthe operating point of the magnetic circuit above the point of inectionof the magnetic flux-magnetic force curve of the magnetic circuit,thereby to prevent an increase in sensitivity when the microphone ispressed against the source of vibrations so strongly as to reduce theair gap below its normal value.

4. The method of maintaining the sensitivity of a contact microphonesubstantially independent of changes in the length of the air gap in themagnetic circuit that result from variations in the steady pressure withWhich the microphone is applied to a source of mechanical vibrations,the magnetic circuit including a permanent magnet in series with the airgap and an armature that is vibrated by the source to alter the flux inthe magnetic circuit, said method comprising magnetizing the permanentmagnet to provide a coercive force that at the normal air gap places theoperating point of the magnetic circuit at the upper level of themagnetic flux-force curve of the magnetic circuit, thereby to prevent anincrease in sensitivity when the microphone is pressed against thesource of vibrations so strongly as to reduce the air gap below itsnormal value.

5. The method of maintaining the sensitivity of a contact microphonesubstantially independent of changes in the length of the air gap in themagnetic circuit that result from variations in the steady pressure withwhich the microphone is applied to a source of mechanical vibrations,the magnetic circuit including a permanent magnet in series with the airgap and an armature that is vibrated by the source to alter the flux inthe magnetic circuit, said method comprising adjusting the coerciveforce of the permanent magnet so that at the normal air gap the magneticcircuit is operated in the region cf saturation of the magnetic circuit,thereby to prevent an increase in sensitivity when the microphone ispressed against the source of vibrations so strongly as to reduce theair gap below its normal value.

6. In the operation of a throat microphone of the moving armature typecomprising a magnetic circuit including an air gap, the method ofreducing the effect on the output of the increase of transmissioneiiiciency of the neck tissues with increasing pressure of application,which comprises adjusting the air gap to a normal value Which preventscomplete closure of the air gap at the maximum pressure of application,and adjusting the coercive force of permanent magnetization to produceincipient saturation in the magnetic circuit, whereby a decrease inoutput is produced by further reduction in the air gap.

'7. In the operation of a throat microphone of the moving armature typecomprising a magnetic circuit including an air gap, the method ofreducing the effect on the output of the increase of transmissioneiiciency oi the neck tissues with increasing pressure of applicationwhich comprises adjusting the air gap to a normal value which preventscomplete closure of the air gap at the maximum pressure of application,and adjusting the coercive force of permanent magnetization to producesaturation in the magnetic circuit whereby a decrease in output isproduced by further reduction in the air gap.

8. In the operation of a throat microphone of the moving armature typecomprising a magnetic circuit including an air gap, the method ofreducing the effect on the output of the increase of transmissioneliciency of the neck tissues with increasing pressure of applicationwhich comprises adjusting the air gap to a normal value which preventscomplete closure of the air gap at the maximum pressure of application,and adjusting the reluctance of the magnetic circuit to produce therein(incipient) saturation whereby a decrease in output is produced byfurther reduction in the air gap.

9. In the operation of a throat microphone of the moving armature typecomprising a magnetic circuit including an air gap, the method ofreducing the effect on the output of the increase of transmissionefficiency of the neck tissues with increasing pressure of applicationwhich comprises adjusting the air gap to a normal value which preventscomplete closure of the air gap at the maximum pressure of application,and adjusting the reluctance of the magnetic circuit to produce thereinsaturation whereby a decrease in output is produced by further reductionin the air gap.

10. In the operation of a throat microphone of the moving armature typeincluding a magnetic circuit having an air gap, the method of reducingthe effect on the output of the increase of transmission eciency of theneck tissues with increasing pressure of application, which methodcomprises increasing the air gap to a normal value which preventscomplete closure of the air gap at the maximum pressure of application,relating the reluctance of the magnetic circuit and the coercive forceof permanent magnetization to produce therein incipient saturation,whereby a decrease in output is produced by further reduction in the airgap.

11. In a throat microphone of the moving armature type comprising amagnetic circuit including an air gap, the method of reducing the effecton the output of the increase of transmission eiiciency of the necktissues with increasing pressure of application which comprisesadjusting the air gap to a normal value which prevents complete closureof the air gap at the maximum pressure of application, and adjusting thereluctance of the magnetic circuit and the coercive force of permanentmagnetization to produce therein saturation, whereby a decrease inoutput is produced by further reduction in the air gap.

12. A contact microphone comprising, in combination, an armature adaptedto be vibrated by vibrations of a source, a ferrous magnetic circuitincluding said armature and an air gap between said armature and a polepiece, the length of the air gap being aiected by the pressure underwhich said microphone is held in contact with said source of vibrations,a coil embracing said magnetic circuit and having a pair of terminalsfor connection to an external electrical circuit, and magnetizing meansoperative at the normal air gap to place the magnetic circuit in a stateof incipient saturation, whereby the output remains substantiallyconstant with variations in the air gap due to variations in thepressure under which said microphone engages said source of vibrations.

13. A contact microphone comprising, in combination, an armature adaptedto be vibrated by vibrations of a source, a ferrous magnetic circuitincluding said armature and an air gap between said armature and a polepiece, the length of the air gap being affected by the pressure underwhich said microphone is held in Contact with said source of vibrations,a coil embracing said magnetic circuit and having a pair of terminalsfor connection to an external electrical circuit, and magnetizing meansoperative at the normal air gap to place the magnetic circuit in a stateof saturation, whereby the output remains substantially constant withvariations in the air gap due to variations in the pressure under whichsaid microphone engages said source of vibrations.

14. A contact microphone comprising, in combination, an armature adaptedto be vibrated by vibrations of a source, a ferrous magnetic circuitincluding said armature and an air gap between said armature and a polepiece, the length of thev air gap being affected by the pressure underwhich said microphone is held in contact with said source oi vibrations,a coil embracing said magnetic circuit and having a pair of terminalsfor connection to an external electrical circuit, and a permanent magnetof such strength that with the normal air gap the magnetic circuit is ina state of saturation, whereby the output decreases with variations inthe air gap due to variations in the pressure under which saidmicrophone engages said source of vibrations.

STUART BALLANTINE.

CERTIFICATE OE CORRECTION., Patent No. 2,165,125. July LL, 1959.,

' STUART BALLANTINE..

It is hereby certified that error eppears in the printed specificationof the above numbered patent requiring correction as follows: Page L1,first colmrin, line 29, claim 8, for (incipient)" read incipient; andthat the said Letters Patent should be read with this correction thereinthat the same may conform to the record of the case in the 'PatentOffice.

Signed and sealed this 8th day of August, A. D. 1959,

Leslie Frazer, (Seal) Acting Commissioner of Patents.

CERTIFICATE OE CORRECTION., Patent No.. 2,165,125. July in, 1959n STUARTBALLANTINE n It is hereby certified that error appears in the printedspecification of the above numbered patent requiring correction asfollows: Page L| first column, line 29, claim 8, for "(incipient)" readincipient; and that the said Letters Patent should be read with this'correctionv therein that the same may conform to the record of the casein the 'Patent Office,

Signed and sealed this 8th day of August, Aa 1).,'1959o Leslie Frazer,(Seal) Acting Commissioner of Patents.

